1. Field of the Invention
The present invention relates to a DA converter that can be applied to a solid-state imaging device represented by a CMOS image sensor, a solid-state imaging device, and a camera system.
2. Description of the Related Art
A current-controlled digital-to-analog (DA) converter (hereinafter referred to as a DA converter) is generally configured to include a plurality of basic current source cells generating a constant current and an output resistor serving as a current-to-voltage converter.
FIGS. 1A and 1B are diagrams showing an exemplary configuration of a current-controlled DA converter having a basic current source cell.
FIG. 1A shows an exemplary basic configuration of the DA converter when a ground potential GND is used as a reference potential, and FIG. 1B shows an exemplary basic configuration of the DA converter when a power supply potential Vdd is used as a reference potential.
The basic current source cells C1-P to Cn-P shown in FIG. 1A each have p-channel MOS (PMOS) transistors P1 to P3 and an output resistor R1.
The basic current source cells C1-N to Cn-N shown in FIG. 1B each have n-channel MOS (NMOS) transistors N1 to N3 and an output resistor R1.
The value of the current flowing in the basic current source cells C1-P to Cn-P or C1-N to Cn-N is determined by a predetermined bias voltage Vbias. Then, a selection transistor pair P2/P3 or N2/N3 supplies the current to the output resistor R1 from the power supply line Vdd or ground line GND which is at the reference potential.
In this way, the current-controlled DA converter generates a desired output voltage Vout by using the output resistor R1 as a current supply destination of the plurality of basic current source cells Cn.
FIG. 2 is a diagram showing a specific example of a DA converter having a reference current generating section and a noise propagation path in a circuit.
The DA converter shown in FIG. 2 corresponds to the configuration shown in FIG. 1A and has a reference current generating section B1.
In many cases, the bias voltage Vbias that determines the current value of the basic current source cells C1-P to Cn-P is generated by a mirror connection established by a current mirror circuit that connects a reference current generated from the constant reference voltage Vref, which exhibits a strong temperature or process variation, via the reference current generating section B1 to the basic current source cells.
Such a current-controlled DA converter is used in an analog-to-digital converter (hereinafter referred to as an AD converter) that converts an analog pixel signal of a solid-state imaging device (image sensor), in which unit pixels are arranged in a matrix form, to a digital signal.
Examples of a solid-state imaging device having this kind of AD converter are disclosed in JP-A-2000-152082, JP-A-2002-232291, and JP-A-2007-059991, for example.
According to the solid-state imaging device disclosed in Patent Documents mentioned above, a voltage comparing section compares an analog pixel signal selected for each line or pixel with a reference voltage (ramp wave RAMP) which varies uniformly for the conversion into a digital signal.
Moreover, in the solid-state imaging device, a counter section performs a count operation concurrently with the comparing operation, and a digital signal corresponding to the pixel signal is obtained based on a count value at the time of completion of the comparing operation.
Specifically, JP-A-2000-152082 discloses an image sensor, JP-A-2002-232291 discloses an analog-to-digital converter and an image sensor using the same, and JP-A-2007-059991 discloses a DA converter, an AD converter, and a semiconductor device.